Wyeth-14643 (WY), a peroxisome proliferator (PP), is a liver carcinogen in rats and mice. However, the potential human carcinogenicity of this chemical is currently uncertain. WY is known to induce the expression of genes involved in peroxisomal fatty acid metabolism as well as DNA synthesis in the liver. Earlier work in this laboratory showed that nonparenchymal cells (NPC) play a role in WY-induced DNA synthesis in rat hepatocytes in vitro. This increase could be inhibited by either 11-undecynoic acid, an inhibitor of P450 4A1, or indomethacin, which inhibits cyclooxygenases (Cox). Present work was initiated to further characterize the respective roles of hepatocytes and NPC in the mechanisms of WY-induced carcinogenicity. Four month-old male B6C3F1 mice were treated with 200 or 100 mg WY/kg body weight by oral gavage for 4 or 12 hr, respectively. Following a two-step collagenase perfusion hepatocytes and NPC were isolated and purified. RNA was isolated from both cell fractions and gene expression was measured by reverse transcriptase polymerase chain reaction. Expression of genes, such as CYP 4A1, the liver fatty acid binding protein (LFABP), and acyl CoA oxidase (ACoAse) was significantly induced in hepatocytes as early as 4 h after dosing. A similar but weaker pattern observed in the NPC fraction was attributed to trace contamination of NPC with hepatocytes. The constitutive Cox 1 enzyme was expressed in NPC but not in hepatocytes and was not induced by WY. Present work also indicated that Cox 2 mRNA was not detectable in NPC or HPC obtained from control or WY-treated mice. However, treatment of mice with WY resulted in increased expression of Cox 2 protein in NPC. These results suggest that the effect of NPC on WY-induced DNA synthesis in hepatocytes may be mediated by fatty acid metabolites formed via Cox enzymes.